At the present time, ball mills of extremely large size are used in the copper industry to reduce ore to a size sufficiently fine to enable it to be refined in a floatation process. Such a ball mill includes as an essential element a drum which is mounted on a horizontal axis and usually is of a diameter ranging from twenty to thirty feet. A large number of balls are contained within this drum and they usually are of steel. Ore which has been partially reduced in size is fed into the drum at one end together with water. As the drum rotates the action of the balls on the ore aggregate achieves the reduction in size of the aggregate.
At the end of the drum remote from the end at which the ore is introduced thereinto, the drum is provided with a cylindrical throat the inner surface of which carries a spiral blade having a reverse pitch whereby some of the balls and aggregate which enter the throat are thrown back into the drum. Mounted on the outer end of this throat and co-axial therewith is a trommel screen which is cylindrical in shape. Ore aggregate of a predetermined size together with water passes through the screen openings at the bottom of the trommel and falls into a hopper from which it is pumped through a conduit to a cyclone separator located just above the ore entry end of the drum. Ore particles of proper size for the floatation process are drawn off the top of the separator while the remaining aggregate and water are passed back through the entry end of the drum where they are recycled.
Due to the grinding action, many of the steel balls become deformed and out of shape. The end of the trommel screen remote from the drum throat is open and these steel balls together with large pieces of aggregate fall out of the open end, and under present day operating conditions, are discarded as waste because there is no known convenient or practical way for returning them to the drum. While these materials may have some value, it is not sufficiently great to warrant attempt to recycle them in the drum.